Electrostatically-addressed MEMS array system and method of use

ABSTRACT

The present invention provides an improved electrostatic micro actuator array system comprising a plurality of electrostatic micro actuators, each of the micro actuators further comprising at least one hold-down electrode and at least two pull-down electrodes positioned to actuate the micro actuator. A hold-down signal line is then coupled to each of the hold-down electrodes of each of the plurality of micro actuators and a plurality of first pull-down signal lines coupled to one of the at least two pull-down electrodes of each micro actuator and a plurality of second pull-down signal lines coupled to another of the at least two pull-down electrodes of each micro actuator, the first pull-down signal lines and the second pull-down signal lines configured in a cross-point matrix such that a unique pair of first pull-down signal lines and second pull-down signal lines is associated with each of the plurality of micro actuators. The system and method of the present invention reduces the number of driving lines required for the micro actuator array. In a particular embodiment, a reconfigurable microelectromechanical (MEMS) micromirror array system capable of deflecting incident light onto or away from a detector is described.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application claims priority to U.S. Provisional Patent ApplicationNo. 60/908,474, filed on Mar. 28, 2007, entitled,“Electrostatically-Addressed MEMS Array”.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under Grant No.DASG60-00-C-0089 awarded by the US Army Space and Missile DefenseCommand. The Government has certain rights in the invention.

BACKGROUND

Measuring the intensity of the light passing through a medium atspecific wavelengths, one can determine the presence and concentrationof various chemical and biological components. Absorption can be causedby a chemical reaction with an added reagent or by direct absorption bythe molecules. Fluorescence can also be a powerful analytical tool,especially in biological detection. Dispersing the light into spectralcomponents in a spectrograph, and then detecting the wavelengths ofinterest using an array of photodetectors can determine the intensitiesat specific wavelengths of interest. Commercial spectrometers using thisprinciple are widely available in the visible range, largely due to theavailability of chemical reagents, and inexpensive arrayedphotodetectors (primarily charge coupled device (CCD) arrays). Aschematic of a traditional detector array-based spectrometer is shown inwith reference to FIG. 1A. In the infrared spectral regions,photodetector arrays are more costly and prone to thermal noise. Toreduce cost or improve sensitivity, several other techniques have founduse in commercial instruments. These include Hadamard TransformSpectrometers, Fourier Transform Spectrometers, and monochromators. BothHadamard and Fourier transform spectrometers require longer measurementtimes and computing power in exchange for improved sensitivity. Themonochromators use mechanically scanned slits or gratings to measure asingle frequency at a time; measurement of a variable passband widthwould require changing slits, and measurement of noncontiguouswavelength bands is not practical. Often analytical techniques canbenefit from instantaneous or time-resolved measurements, makingserially read detector arrays and the aforementioned time-integratedtechniques less relevant.

In a different type of spectrometer, one can use a programmable mirrorarray to direct only the desired wavelengths onto a single detector.MEMS mirrors can allow rapid reconfiguration of the desired spectralpassbands, and the detector may be selected based on cost, sensitivity,response speed requirements, etc. This layout would be especially usefulin industrial, consumer, and military applications where an instrumentis needed to make repeated measurements (e.g. absorption at a specificwavelength) of fixed spectral bands, while allowing fast andre-programmable configuration. Such a system is illustrated in FIG. 1B.The benefit of this type of spectrometer is that only the wavelengths ofinterest are measured, and since the number of detectors is reduced, itcan be a more specialized type. For example, a fast sensitive detectorcould be used to provide time-resolved fluorescence measurements.

One of the primary challenges for making such a system is areconfigurable micromirror array capable of deflecting incident lightonto or away from a detector. Accordingly, what is needed in the art isan improved system and method for the control of a MEMS-fabricatedelectrostatic array.

SUMMARY

The present invention provides an improved electrostatic micro actuatorarray system comprising a plurality of electrostatic micro actuators,each of the micro actuators further comprising at least one hold-downelectrode and at least two pull-down electrodes positioned to actuatethe micro actuator. A hold-down signal line is then coupled to each ofthe hold-down electrodes of each of the plurality of micro actuators anda plurality of first pull-down signal lines coupled to one of the atleast two pull-down electrodes of each micro actuator and a plurality ofsecond pull-down signal lines coupled to another of the at least twopull-down electrodes of each micro actuator, the first pull-down signallines and the second pull-down signal lines configured in a cross-pointmatrix such that a unique pair of first pull-down signal lines andsecond pull-down signal lines is associated with each of the pluralityof micro actuators.

In an additional embodiment, the present invention provides a method forreducing the number of electrostatic drive lines required for theconfiguration of an array of electrostatic micro actuators. In oneembodiment, each of the micro actuators further includes at least onehold-down electrode and at least two pull-down electrodes positioned toactuate the micro actuator. The method of the present invention includesthe steps of positioning a hold-down signal line coupled to each of thehold-down electrodes of each of the plurality of micro actuators,positioning a plurality of first pull-down signal lines coupled to oneof the at least two pull-down electrodes and a plurality of secondpull-down signal lines coupled to another of the at least two pull-downelectrodes, the first pull-down signal lines and the second pull-downsignal lines configured in a cross-point matrix such that a unique pairof first pull-down signal lines and second pull-down signal lines isassociated each of the plurality of micro actuators, applying a voltageto the hold-down signal line, applying a voltage to the unique pair offirst pull-down signal lines and second pull-down signals for one of theplurality of micro actuators such that the summation of the voltages onthe hold-down signal line and the at least two pull-down signal lines issufficient to actuate the micro actuator to a pulled-down state andremoving the voltage on the at least two pull-down electrodes whilemaintaining the voltage on the hold-down electrode to hold the microactuator in the pulled-down state.

In a particular embodiment of the present invention, a reconfigurablemicroelectromechanical (MEMS) micromirror array system capable ofdeflecting incident light onto or away from a detector is provided. Themicromirror array of the present invention includes a plurality ofcantilever-mirror elements and each cantilever-mirror element includesan electrostatically actuated cantilever, a mirror coupled to theelectrostatically actuated cantilever through at least one hingeelement, a hold-down electrode positioned underneath the cantilever, andat least two pull-down electrodes positioned underneath the cantilever.With this embodiment, a reduced number of drive lines can be used toconfigure the mirrors of the array as previously described.

The present invention describes the design and control ofMEMS-fabricated electrostatic arrays. The MEMS structures can befabricated using any number of surface micromachining processes, ofwhich one possibility is described. A multiplexing method is introducedin the design to enable positioning a large number of devices from a fewelectrical inputs, which is necessary for practical applications whenintegrated control circuitry cannot be created on-chip with the MEMSdevices. This approach also enables separate optimization of theactuation and control sections, and significantly reduces the number ofdrive signals required.

The present invention addresses the problem of realistically addressinglarge arrays of electrostatically-actuated MEMS structures. The methodreduces the need for high-density wiring and custom application specificintegrated circuitry (ASICs) to be integrated into the MEMS processing,either through monolithic or flip-chip assembly techniques, by reducingthe number of actuation signals required.

In a particular embodiment, an application in an optical spectrometerinstrument is illustrated. Toward reducing the cost, size and complexityof the mirror arrays needed, the design, fabrication, and control of aMEMS-actuated mirror array that can be coupled with a conventional orMEMS-fabricated spectrograph and one or two photodetectors forspectrometry detection applications is presented. The surfacemicromachined parts need not lie in a line, so they could be efficientlymatched with the output spectrum of spectrographs without flat-fieldcorrection. In addition to the spectrometer applications, thereconfigurable mirrors could find application in the telecommunicationsfield. The operating principles of the MEMS mirror actuators could alsobe useful for other MEMS applications where a large number of parts needto be configured using only a few electrical drive lines.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made tothe following detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1 is an illustration showing a comparison of a prior art (A)traditional detector-array based spectrometer and a prior art (B) mirrorarray and detector based spectrometer.

FIG. 2 is an illustration of a solid model of pop-up mirror structureshowing hinged cantilever and mirror structures and various designfeatures in accordance with an embodiment of the present invention.

FIG. 3 is a series of illustrations showing simulated deflection ofsample test structures using a constant force applied to the cantilevertop surface and three fulcrum structures in accordance with variousembodiments of the present invention: (A) single torsion hinge=2.8degrees deflection, (B) torsion and flexible hinge structures=5.9degrees, and (C) flexible hinge structure with a vertically-constraineddimple point=7.8 degrees.

FIG. 4 is a series of illustrations showing an example of force versuselectrode gap for (A) simplified moveable parallel plate capacitorstructure with initial gap d_(o) and voltage V applied across theplates, for the instances: (B) single lower electrode with V=30V, (C) 3lower electrodes with 50%, 30% and 20% of the area of case (B), andV=40V.

FIG. 5 is a diagram for a design for addressing 56 mirrors with 17electrodes by using the electrostatic hysteresis effect and electrodemultiplexing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The spectral resolution of a spectrometer is a function of manyinstrument parameters, such as entrance aperture size, grating size andperiod, detector element size, and optical aberrations. Although oftennot the limiting factor, a smaller detector element can yield higherresolution. Typical arrayed detector elements are 14-50 μm in width;when coupled with a typical ⅛ m compact spectrograph and 600 line/mmgrating, resolution better than 2 nm is achievable at 1100 nm centerwavelength. Similar performance is expected when replacing theconventional spectrometer's detector array with a programmable mirrorarray and a detector.

Several mechanisms are commonly used for MEMS actuators: thermal,electrostatic, magnetic, and piezoelectric. Thermal and magneticactuators are unsuitable for low power operation unless the devices canbe configured and held into a state with another means, andpiezoelectric actuators commonly use non-standard fabrication materials(e.g. lead) not allowed in many MEMS or IC fabrication facilities. Thepresent invention relies on electrostatic mechanisms for the MEMSactuators.

In a particular embodiment, the mirror devices of the present inventionwere designed and fabricated using a silicon nitride structural-layerMEMS process, summarized herein. A silicon nitride passivation layer andTi/Au are deposited on a silicon wafer. The Ti/Au is then patterned witha metal0 mask that defines lower-layer electrical routing andhigh-adhesion anchor locations for the latter structural devices. Asacrificial polymer is then deposited and patterned using photoresistand developer, creating anchor points through the polymer film. A Au/Timetal1 film is then sputtered atop the wafer, conformally filling intothe anchor holes. The areas where the anchored metal1 is atop metal0gold creates a strong Au—Au interface. However, since gold does notadhere well to silicon nitride, structures anchored to the exposedpassivation layer have poor adhesion. This allows the design ofvertically constrained dimple points the full depth of the sacrificiallayer without a separate masking step. A holemetal1 step is used toelectrically isolate features (eg. for making electrical switches,routing, etc.), and also to create unmetallized submicron indentationsinto the sacrificial layer, which can be used to prevent parts fromsticking down when actuated. A layer of structural silicon nitride isgrown by low-temperature (250° C.) PECVD, followed by a sputtered Ti/Aumetal2 film. The metal2, structure, and metal1 films are then patternedand etched by a single mask, leaving a stacked mechanical structure withelectrically conductive metal on both surfaces of the structures. Theexposed reflective metal2 Au on top of all structures is used formirrors in this infrared spectrometer application, and can also be usedfor a metal-metal lid-sealing area for final packaged devices. Thesacrificial polymer is removed by isotropic O₂ plasma etching. In aparticular embodiment, the array utilizes the following non-optimizedlayer thicknesses: metal0=0.3 μm, sacrificial polymer layer=2.4 μm withunmetalized indentations=0.6 μm into the polymer, and the metal1 (0.3μm)+silicon nitride (1.4 μm)+metal2 (0.2 μm) structure stack=1.9 μm.

The electrostatic forces are relatively weak, decreasing with the squareof the electrode distance. Therefore, to increase the mirror deflectionangle, a levered cantilever-mirror actuator is utilized. In suchstructures, a lever is fabricated using an electrostatically actuatedcantilever connected to a mirror through a flexible hinge structure.Small displacement of the cantilever tip (up to the sacrificial materialthickness) can produce a larger out-of-plane angular mirror changecaused by a fulcrum point designed between cantilever tip and mirror.The advantage of this method is that no sliding hinges are required, anda relatively large out-of-plane mirror angle can be achieved withmoderate drive voltages. For spectrometer applications, only a fewdegrees angular movement of the mirrors is needed to spatially separateportions of the spectrum. For example, an f/22 optical spectrographwould require only a 2.6 degree mirror movement to spatially separatethe wanted and unwanted portions of the output spectra.

In accordance with a particular embodiment of the present invention, thearray includes a ground plane under the mirrors to prevent stray chargesfrom causing spurious electrostatic forces, which can pull theelectrically grounded mirror downward. Additionally, submicronindentations are included at several points under each cantilever toprevent metal-metal contact welding, and to create a low contact area toreduce the chance of stiction which can cause the parts to stay stuckdown even after turning off the actuation voltages. In an additionalembodiment, vertically-constrained dimple fulcrum points are included.This dimple was found to be beneficial since unwanted verticaldisplacement of the torsion hinge reduces the maximum angularmagnification of the cantilever structure. A solid model of the micromirror 10 of the present invention showing various design features of acantilever-mirror element is accordance with an embodiment of thepresent invention are illustrated in FIG. 2 in which an elecrostaticallyactuated cantilever 20 is coupled to a micro mirror 15 through aflexible hinge 25 and/or a torsion hinge 30. In this particularembodiment a dimple fulcrum is implemented in the torsion hinge 35. Thecantilever further includes a cantilever attachment 40.

FIGS. 3A-C show a comparison of Finite Element Analysis (FEA)-simulatedmirror angular displacement for the case of a simulated electrostaticpressure applied to the 100×88 μm cantilever, for designs with: 3A) asingle torsion hinge=2.8 degrees, 3B) torsion and flexible hingestructures=5.9 degrees, and 3C) flexible hinge structure with a dimplefulcrum point=7.8 degrees. A constant average pressure (0.1 MPa) wasapplied to the surface of the cantilever in the simulations, instead ofperforming coupled electrical-mechanical simulations. This allowedcomparison of various hinge designs while reducing simulation time. Inthe figures, the vertical dimension is exaggerated to more clearly showdisplacement. It can be noted that in the designs without the dimplefulcrum point (FIGS. 3A, 3B), work is undesirably performed to bend thetorsion beam downward. This vertical deflection also reduces the usefuldisplacement of the lever arm, so the maximum angle achievable on themirror before the cantilever snaps down will be reduced. Indeed, theforce applied in FIG. 3B already exceeds the force required to push thecorners of the upper plate to the substrate (i.e. the tip of thecantilever is unrealistically pulled below the surface of the wafer).

In accordance with the present invention, it is desirable to position alarge number (N) of mirrors into one of two states (i.e. rotated up orremaining flat) so the incident spectrum can be selectively deflectedonto a detector. In the simplest layout, the designer could use oneelectrode per mirror to configure each. However, if the number ofmirrors is large (for example 1024) and on-chip drive electronics androuting are unavailable, the task of connecting the individuallyaddressed mirror array to external electronics becomes a wire-bonding(or ball-grid connect) challenge. Additionally, since electrostaticdevices typically operate at voltages above CMOS-levels, amplificationcircuitry is usually required, adding to the impracticality of thissolution. Thus, it is desired to develop a method to reduce the numberof connections and amplifiers required to configure the mirror array.The electrostatic drive principle in accordance with the presentinvention is summarized in the following text, and a solution forreducing the number of driving lines based on the principles ispresented.

Electrostatic MEMS actuators have been used in a wide variety ofdevices. They operate on the principles of electrostatics wherein aforce is exerted on oppositely charged parallel plates. As illustratedin FIG. 4A, upon application of a voltage between the plates, the lessconstrained moveable plate is pulled toward the other by anelectrostatic force, F_(e):

$\begin{matrix}{F_{e} = {\frac{ɛ\; A}{2d^{2}}V^{2}}} & (1)\end{matrix}$where ε is the permittivity of the (air) gap, d is the plate spacing, Ais the electrode area, and V the applied voltage. If the moveableelectrode plate is attached to a fixed structure through a spring, thedirection of the restoring mechanical force (F_(m)) is opposite that ofthe electrostatic force and proportional to the displacement:F _(m) =k(d _(o) −d)  (2)where d_(o) is the initial plate position and k is the spring constant(related to the Young's modulus of the material and the moment ofinertia of the plate). The equilibrium position occurs when F_(m) andF_(e) are equal, or when F_(e) exceeds F_(m) and the plates are pulledinto contact with each other. It should be noted that F_(e) isproportional to the inverse square of the electrode gap, whereas F_(m)is linearly proportional to the reduction in inter-electrode gap fromthe initial position. This is illustrated in the hypothetical exampleshown in FIG. 4B (100×100 μm plates, initially separated by a distanced_(o)=2 μm, with 30V applied, k=50 μN/μm). The moveable plate is pulleddownward from d=2 μm by the larger electrostatic force, until theequilibrium gap (d˜1.7 μm) is reached because F_(e)=F_(m). The solutionnear 0.8 μm is an unstable solution, since a small increase in gapcauses the larger mechanical force to continue increasing the plateseparation until the stable solution near 1.7 μm is reached. However, ifthe initial inter-electrode gap can be reduced below the secondintersection point at d˜0.8 μm, then F_(e) exceeds F_(m), and theequilibrium point will be at gap d=0 (i.e. the plates are in contact).This dual-equilibrium point operation results in a hysteresis effectthat can be used to the designer's advantage, as discussed below.

If instead of using a single fixed electrode under each element of theelectrostatic actuator array, three electrodes are used under eachactuator, a reduction of the total number of driving lines can berealized. When a voltage is applied to an electrode, each generates anelectrostatic force on the moveable electrode, as per Eq. (1). If asufficient voltage is applied, the sum of the forces (F_(e(sum))) can belarge enough to pull down the moveable plate to the fixed electrode, asillustrated in FIG. 4C. The figure shows an example where the threeelectrode areas are 50%, 30%, and 20% of the area of the 100×100 μmplate used in FIG. 4B, and V=40V. If fewer than all three electrodeshave voltage applied to them, the pull-in force is not exceeded, and theupper plate will deflect slightly to a position over the lowerelectrode. If, however, 40V is applied to all three electrodes,F_(e(sum)) will always exceed F_(m), causing the moveable plate to pulldown to d=0. After being pulled down, the plate can be held in place bykeeping just a single electrode active, due to the electrostatichysteresis effect mentioned previously. We can designate one of theelectrodes (for example, the one resulting in F_(el)) as a hold-downelectrode, and the other two electrodes as pull-down electrodes.

With reference to FIG. 5, the method for the reduction of driving linesin accordance with the present invention is described. In thisembodiment, the micro mirror includes a hold-down electrode 45, a firstpull-down electrode 50 and a second pull-down electrode 55 as previouslydescribed. In a particular embodiment, a single line is designated asrunning under an area of all actuators as the hold-down signal 60. Thepull-down lines are then configured similar to a cross-point matrix usedin memory cells. One of ‘L’ vertical bus lines 65 is designed in an areaunder each of a group of ‘L’ actuators, and repeated in ‘M’ groupings70. Each one of ‘M’ horizontal lines are designed in an area under oneentire group of ‘L’ actuators. There is then a unique pair of ‘L’ and‘M’ lines that will designate each actuator. Thus to configure theactuator array with any specified set of actuators down, the controllercan maintain a voltage to the hold-down signal 60, then sequentiallychoose the combinations of electrodes L 65 and M 70 to pull down theseactuators. The hold-down electrode serves to keep the already-programmedactuators in their pulled-down state, and can be turned off to allowresetting of the array for re-programming. An additional ground signalline 75 is coupled to the top of the cantilever.

The total number of electrodes needed to configure L*M actuators isL+M+2 (L+M pull-down electrodes+1 hold-down electrode+1 electrodeconnected to the top actuator). For the example shown in FIG. 5, withL=7 and M=8, 56 actuators attached to hinged minors can be configuredwith 17 lines, compared to 57 required if one electrode was used peractuator. Only 66 actuators (L=M=32) would be needed to configure 1024actuators, which is a reasonable number of drive signals out of an ASICor FPGA in a standard package. Though the current design uses only 2pull-down electrodes and 1 hold-down electrode under each actuator,additional savings could be realized by using more electrodes (forexample, using 3 pull-down electrodes per actuator, just 26 electrodeswould be required to configure 512 actuators). There is likely apractical limit to the number of pull-down electrodes, due tofabrication tolerances and minimum design rule spacing.

Several variants of the pop-up minor devices were fabricated and tested.The design variations included cantilever length (100 μm or 150 μm);straight or Z-shaped cantilever attachment structures; and dimplefulcrum points with flexible hinges, or torsion hinges. Each variationof the current design includes a flexible hinge to reduce the high drivevoltage requirements seen in previous designs without the flexiblehinges.

For testing the various design styles and verifying the electrostaticmultiplexing concept, a Wyko NT3300 white-light interferometer profilerwas used to provide non-contact measurements of device topography. Thestandard instrument base has been modified to act as a probe station andincludes a platen to hold several microprobes for applying voltages tothe test parts. The system is currently limited to 4 probes at a time.Due to this limitation, the electrostatic pull-down and hold-downtesting was performed on individual mirrors, instead of the fullmultiplexed array shown in FIG. 5.

In the experimental results, the pop-up mirror styles with 150 μmcantilevers had a lower pull-in voltage compared to 100 μm cantilevers,due to the larger electrostatic force generated by the former. However,the maximum pull-in mirror deflection angle was greatest (4.5°, at 80V)for the devices with 100 μm cantilevers, fulcrum dimples, and straightcantilever attachments. The design with 150 μm long cantilevers, fulcrumdimples, and straight cantilever attachments achieved only a 3.3° minordeflection (at 65V). The deflection angle reduction is apparently due toslightly more bowing up of the less-rigid longer cantilever near thefulcrum point. The style with 150 μm long cantilevers, fulcrum dimplesand Z-shaped cantilever attachments produced a 3° minor deflection at50V. The largest deflection angle for the styles with only a torsion bar(no fulcrum dimple) was only 1.3°, and was limited by undesirablevertical deflection of the torsion bar.

The electrostatic multiplexing was verified by applying voltage to thethree bottom electrodes, combined or independently. Since the hold-downelectrode is toward the end of the cantilever (see FIG. 5), and has thelargest moment arm (for styles without fulcrum dimple hinges), it hadthe greatest effect. For the 150 μm long cantilever style, with Z-shapedcantilever attachments with torsion and flexible hinges, the pull-involtage for the hold-down electrode was 65V. Because of the presence ofthe 0.6 μm deep indentations under the cantilever, thecantilever-electrode gap cannot be less than 0.6 μm, and there is afinite voltage (50V) where the hold-down force no longer keeps thepulled-down cantilever down. Thus, operating at 60V, within this usablehysteresis range (50-65V), the pull-down electrodes were then applied.As the Pull-down1 electrode was applied, it was noted the pull-involtage was 20V. Releasing the minor, then operating slightly belowpull-in, at V_(hold-down)=60V, V_(pull-down1)=19V, the cantilever boweddown, but did not pull in. When 60V was applied to the Pull-down2electrode, the cantilever snapped down. While keeping the hold-downelectrode at 60V, both pull-down electrodes could be set to 0V, and thecantilever maintained its pulled-down state. Turning the hold-downvoltage below 50V released the cantilever, to allow re-programming ofits state. These results above are summarized in Table I, and show themultiplexing concept as viable.

TABLE I Sequential operation of multiplexer-actuated mirrors Voltage atVoltage at Voltage at Pull Pull hold down down1 down2 Top Time.electrode electrode electrode electrode Result 1 0 0 0 Gnd Mirror homeposition 2 65 0 0 Gnd Pulls down 3 50 0 0 Gnd Releases back to the homeposition 4 60 20 0 Gnd Pulls down 5 60 0 0 Gnd Remains held down 6 50 00 Gnd Releases back to the home position 7 60 19 60 Gnd Pulls down 8 600 0 Gnd Remains held down

Experimental results show that, the mirror arrays with dimpled fulcrumpoints resulted in larger angular movement of the pop-up minors thanversions with a torsion bar fulcrum point. The flexible hinge reducedthe pull-in voltage from previous fabricated devices. The addition ofZ-shaped cantilever attachment points was beneficial for reducingactuation voltage. Further modification to include an even morecompliant attachment mechanism could likely further reduce the actuationvoltage. Because bowing of the cantilever near the fulcrum point wasnoted, an optimized design of flexible hinge and fulcrum dimple couldprovide a larger angular movement, if needed. Reducing the depth of theindentation features under the cantilever would also increase themaximum angular movement of the mirror, since the smallercantilever-electrode gap would produce a larger force; it wouldadditionally reduce the voltage required to maintain the hold-downstate. This change could be easily made in the fabrication process byreducing the etch time at the indentation formation step. Forspectrometer applications, the current angular movements are likelyacceptable. For example, an f/22 optical spectrograph would require onlya 2.6 degree mirror movement to spatially separate the wanted andunwanted portions of the output spectra. If two optical spectral bandsneed to be simultaneously measured, the minor arrays can be abuttedback-to-back, with each array redirecting the light of interest onto aseparate detector.

The multiplexing concept, where electro-mechanical hysteresis is usedand several electrodes contribute electrostatic force, can be used toactuate and hold down arrays of electrostatic actuators. Devices withmultiplexing electrode interconnection have been designed, and asignificant reduction in the number of actuation lines has beenrealized, compared to the 1-electrode-per-minor configuration. Becausethe moment arm of the Pull down2 electrode (FIG. 5) is small compared tothe hold-down and other pull-down electrodes, a modified designincorporating both pull-down electrodes under the cantilever fromattachment point to hold-down electrode would be beneficial to equalizetheir electromechanical contribution.

It is anticipated the switching speed of these devices would beapproximately the same, allowing a complete spectrometer arrayreconfiguration time of less than 1 second.

A MEMS mirror array for use in optical spectrometers has been presented.A scheme for reducing the number of drive electrodes to a practicalnumber has been devised and tested. The scheme includes a hold-downelectrode to maintain the programmed state, and two (or more) pull-downelectrodes to additively achieve pull in of the electrostaticcantilever. If only two of the three electrodes are applied, pull-in isnot achieved. The implemented control methods are useful for the statedapplication, and potentially for others where configuration of arrays ofelectrostatic parts is required.

In a particular embodiment, the devices were fabricated using a siliconnitride surface micromachining process resulting in reflective goldmirror surfaces. Low-adhesion Au-to-silicon nitride dimples were used inthe designs to prevent vertical deflection of the fulcrum point of thecantilever-mirror levered structure. This was shown to produce a largerminor movement compared to the designs using a more conventional torsionbar fulcrum point.

Additional reduction of the drive voltage and increased minor scan anglecould be achieved through the use of thinner structural silicon nitride,more compliant cantilever beam attachment features, or reduced featuredimensions. Use of alternate materials (for example, aluminum) couldallow the mirror array designs to be used in visible opticalspectrometer applications as well.

REFERENCES

-   S. Kedia, S. Samson, A. Farmer, M. C. Smith, D. Fries, S. Bhansali,    “Handheld interface for miniature sensors,” SPIE International    Symposium of Smart Materials, Nano- and Micro-Smart Systems v.    5649 p. 241-252, 2004.-   Alan G. Marshall ed., Fourier, Hadamard, and Hibert Transforms in    Chemistry, New York: Plenum Press, 1982.-   Polychromix. Wilmington, Mass. 01887. http://www.polychromix.com.-   E. Wagner, B. Smith, S. Madden, J. Winefordner, and M. Mignardi,    “Construction and evaluation of a visible spectrometer using digital    micromirror spatial light modulation,” Applied Spectroscopy, v.    11, p. 1715-1719, 1995.-   K. I. Tarasov, The Spectroscope, New York: John Wiley & Sons, p.    77-81, 1974.-   S. Samson, R. Agarwal, S. Kedia, W. Wang, S. Onishi, and J.    Bumgarner, “Fabrication processes for packed optical MEMS devices,”    Proc. ICMENS 2005 Banff, Alberta, Canada, p. 113-118, 2005.-   D. Lopez, F. Pardo, V. Aksyuk, M. Simon, H. Shea, D. Marom, D.    Neilson, R. Cirelli, F. Klemens, W. Mansfield, L. Fetter, E.    Bower, J. Miner, and T. Sorsch, “MEMS minor array for a wavelengths    selective 1×K switch,” Proc. SPIE Smart Sensor, Actuators, and    MEMS, v. 5116 p. 445-455 2003.

The disclosure of all publications cited above are expresslyincorporated herein by reference, each in its entirety, to the sameextent as if each were incorporated by reference individually.

It will be seen that the advantages set forth above, and those madeapparent from the foregoing description, are efficiently attained andsince certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatters contained in the foregoing description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween. Now that theinvention has been described,

1. An electrostatic micro actuator array system, the system comprising:a plurality of electrostatic micro actuators fabricated on a substrate,each of the micro actuators further comprising at least one movableelectrode, at least one fixed hold-down electrode positioned on thesubstrate and underneath the at least one movable electrode and at leasttwo fixed pull-down electrodes positioned on the substrate andunderneath the at least one movable electrode; a hold-down signal linecoupled to each of the fixed hold-down electrodes of each of theplurality of micro actuators; a plurality of first pull-down signallines coupled to one of the at least two fixed pull-down electrodes ofeach micro actuator and a plurality of second pull-down signal linescoupled to another of the at least two fixed pull-down electrodes ofeach micro actuator, the first pull-down signal lines and the secondpull-down signal lines configured in a cross-point matrix such that aunique pair of first pull-down signal lines and second pull-down signallines is associated with each of the plurality of micro actuators. 2.The system of claim 1, further comprising a micro actuator selectioncontroller coupled to the hold-down signal line, the plurality of firstpull-down signal lines and the plurality of second pull-down signallines.
 3. The system of claim 2, further comprising a voltage sourcecoupled to the micro actuator selection controller.
 4. The system ofclaim 1, further comprising a ground plane positioned underneath themicro actuator to prevent stray charges from causing spuriouselectrostatic forces.
 5. The system of claim 1, wherein the movableelectrode of the electrostatic micro actuator is electrically grounded.6. The system of claim 1, wherein the electrostatic micro actuator is anelectrostatically-actuated cantilevered micro mirror.
 7. A method forreducing the number of electrostatic drive lines required for theconfiguration of an array of electrostatic micro actuators, each of themicro actuators fabricated on a substrate and each of the microactuators further comprising at least one movable electrode, at leastone fixed hold-down electrode positioned on the substrate and underneaththe at least one movable electrode and at least two fixed pull-downelectrodes positioned on the substrate and underneath the at least onemovable electrode, the method comprising: positioning a hold-down signalline coupled to each of the fixed hold-down electrodes of each of theplurality of micro actuators; positioning a plurality of first pull-downsignal lines coupled to one of the at least two fixed pull-downelectrodes and a plurality of second pull-down signal lines coupled toanother of the at least two fixed pull-down electrodes, the firstpull-down signal lines and the second pull-down signal lines configuredin a cross-point matrix such that a unique pair of first pull-downsignal lines and second pull-down signal lines is associated each of theplurality of micro actuators; applying a voltage to the hold-down signalline; applying a voltage to the unique pair of first pull-down signallines and second pull-down signal lines for one of the plurality ofmicro actuators such that the summation of the voltages on the hold-downsignal line and the at least two pull-down signal lines is sufficient toactuate the movable electrode of the micro actuator to a pulled-downstate; removing the voltage on the at least two pull-down electrodeswhile maintaining the voltage on the hold-down electrode to hold themovable electrode of the micro actuator in the pulled-down state.
 8. Themethod of claim 7, further comprising removing the voltage on thehold-down signal line to reconfigure the array.
 9. The method of claim7, further comprising applying a voltage to another unique pair of firstpull-down signal lines and second pull-down signals for one of theplurality of micro actuators such that the summation of the voltages onthe hold-down signal line and the at least two pull-down signal lines issufficient to actuate the movable electrode of the micro actuator to apulled-down state.
 10. The method of claim 7, wherein the movableelectrode of the electrostatic micro actuator is anelectrostatically-actuated cantilevered micro mirror.